Costs and benefits of photosynthetic light acclimation by tree seedlings in response to gap formation

Abstract

Some shade leaves increase their photosynthetic capacity (Pmax) when exposed to a higher irradiance. The increase in Pmax is associated with an increase in chloroplast size or number. To accommodate those chloroplasts, plants need to make thick leaves in advance. We studied the cost and benefit of photosynthetic acclimation in mature leaves of a tree species, Kalopanax pictus Nakai, in a cool-temperate deciduous forest. Costs were evaluated as the additional investment in biomass required to make thick leaves, while the benefit was evaluated as an increase in photosynthetic carbon gain. We created gaps by felling canopy trees and examined the photosynthetic responses of mature leaves of the understorey seedlings. In the shade, leaves of K. pictus had vacant spaces that were not filled by chloroplasts in the mesophyll cells facing the intercellular space. When those leaves were exposed to higher irradiance after gap formation, the area of the mesophyll surface covered by chloroplasts increased by 17% and Pmax by 27%. This increase in Pmax led to an 11% increase in daily carbon gain, which was greater than the amount of biomass additionally invested to construct thicker leaves. We conclude that the capacity of a plant to acclimate to light (photosynthetic acclimation) would contribute to rapid growth in response to gap formation.

Notes

Acknowledgements

We thank the staff members at Tomakomai Experimental Forest for their technical support and the experimental set-up, T. Koike and T. Kohyama for generous offer of instruments and S. Kitaoka, S. Kosuge, Y. Onoda and E. Nabeshima for experimental supports, advice and discussion. This work was financially supported in part by Grants-in-aid of the Japan Ministry of Education, Culture, Sports, Science and Technology and by JSPS Research Fellowship for Young Scientists. We declare that the experiments comply with the current laws of Japan.